Traveling-wave tube



Sept. 17, 1957 s. sENslPER TRAvELING-WAVE TUBE 3 SheetsFSheet 1 FiledDec. 8. 1954 Sept. 17, 1957 s. sENslPER 2,806,972

TaAvELING-WAVE TUBE Filed Dec. 8, 1954 3 Sheets-Sheet 2 M M i z ,VH a fa SCPL 17, 1957 s. sENslPER 2,806,972

TRAvELING-WAVE TUBE 3 Sheets-Sheet 3 Filed Deo. 8. 1954 VMM PatentedSept. 17, 1957 TRAVELlNG-WAVE TUBE Samuel Sensiper, Los Angeles, Calif.,assignor to Hughes Aircraft Company, Culver City, Calif., a corporationof Delaware Application December 8, 1954, Serial No. 473,856

Claims. (Cl. 315-3.5)

This invention relates to micro-wave tubes and more particularly to atraveling-wave tube incorporating a slow- Wave circuit constituting aperiodic waveguide device having unidirectional or selective attenuationcharacteristics.

ln the conventional traveling-wave tube, a slow-wave structure isgenerally employed to propagate an electromagnetic signal wave along thepath of an electron stream at a velocity to effect interaction betweenthe electromagnetic wave and the electron stream. In that this type oftube provides gain over a broad range of frequencies, it is verydifficult to provide an output circuit with an impedance that matchesthat of the slow-wave structure over this entire range of frequencies.This results in a portion of the energy of the electromagnetic wavesappearing at the output circuit being reflected and being propagatedback towards the input circuit by the slow-wave structure. When thisreflected portion of the output wave arrives at the input circuit, aportion of its energy may again be reflected and propagated along withthe electron stream towards the output circuit. This last reflectedportion of the electromagnetic wave is progressively amplified along itsreturn path. When this reflected portion is amplified to the extent thatit has a magnitude greater than that of the initial Wave appearing atthe output circuit, the tube is said to be in self-oscillation. Thus, itis apparent that a traveling-wave tube may self-oscillate at anyfrequency where the gain is greater than the reflection and propagationlosses through the tube.

In accordance with the present invention, a travelingwave tube isprovided wherein the tendency to selfoscillate is substantiallyeliminated whereby gain and power output may be increased.Self-oscillations are eliminated by employing a periodic waveguidedevice having unidirectional or selective attenuation characteristicsfor the slow-wave structure of the tube. These unidirectional orselective attenuation characteristics are achieved by utilizing thegyroresonance properties of ferrite materials in conjunction with themagnetic field normally employed for constraining the electron stream.Properties of ferrite materials of the gyroresonance type are describedin an article entitled, Ferrites in microwave applications, by J. H.Rowen, which appears on pages 1333-1369 of the Bell System TechnicalJournal for November 1953, published in New York.

In a first embodiment of the invention, a folded waveguide structure isemployed to propagate an electromagnetic wave along a path through thecentral portion of each waveguide section. In order to provide thisstructure with unidirectional attenuating characteristics, slabs offerrite material are disposed within the sections of the waveguidenormal to the path of the electron stream, the successive slabs beingspaced from the wall of the guide on alternate sides of the electronstream. A single longitudinal magnetic field is then developed to directthe electron stream along the path and to produce gyroresonance withinthe slabs of ferrite material at frequencies where `the unidirectionalattenuating characteristics are desired. The frequency at whichgyroresonance with its concomitant maximum attenuation occurs depends onthe magneti-c field Within the ferrite material. Thus, the frequenciesat which unidirectional attenuation is desired may be staggeredthroughout the band wherein gain is provided by controlling the magneticfield within the individual ferrite slabs. In that the samemagnetomotive force energizes all the ferrite slabs, the magnetic fieldthrough any one slab may be determined by its thickness or lessened by amagnetic shunt. In addition, it may be desirable to progressivelyincrease the coupling of the ferrite slabs to the wave being attenuatedso as to distribute the heat dissipation uniformly over all of theslabs. This may be accomplished by progressively increasing the spacingbetween the ferrite slabs and the wall of the waveguide.

An alternative embodiment of the present invention is similar to thatdescribed above except that the ferrite slabs are spaced contiguous tothe wall of the guide so as to attenuate waves propagated by the guideequally in both directions. This attenuation, however, is effected onlyfor frequencies on both sides of the operating frequency range bysuitable energization of the ferrite material by the electron streamfocusing field. In this manner both forward and backward waves areattenuated at frequencies adjacent the operating range whereselfoscillations are likely to occur.

Another embodiment of the present invention employs a helical waveguidecoupled to an electron stream by appropriate openings at either thecenter or at one side thereof. A longitudinal magnetic field is used forboth focusing the electron stream and for developing unidirectionalattenuating characteristics in the waveguide structure by simultaneouslyenergizing a ribbon helix constituted of ferrite material disposedparallel to and spaced from an inner wall of the guide. This structurehas the advantage in that it does not have any discontinuities todetrimentally reflect portions of the electromagnetic wave beingpropagated along the path of the electron stream.

It is therefore an object of the present invention to provide animproved traveling-wave tube wherein the tendency to self-oscillate isminimized.

Another object of the invention is to provide a travelingwave tubeincorporating a slow-wave structure having unidirectional attenuatingcharacteristics.

Still another object of the invention is to provide an improvedapparatus employing ferrite materials in conjunction with the electronstream focusing field to effect unidirectional attenuation in the waveguiding structures of traveling-wave tubes.

A further object of the present invention is to incorporate a pluralityof ferrite slabs in a traveling-wave tube to effect unidirectionalattenuation wherein substantially equal quantities of heat aredissipated in each ferrite slab.

A still further object of the present invention is to provide atraveling-wave tube having selective bidirectional attenuatingcharacteristics adjacent its operating frequency range.

The novel features which are believed to be charactertic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings in which several embodiments of the invention areillustrated by way of example. It is to be expressly understood,however` that the drawings are for the purpose of illustration anddescription only, and are not intended as a definition of the limits ofthe invention.

Fig. l is a diagrammatic sectional view of an embodiment of thetraveling-wave tube of the present invention together with associatedcircuitry;

Figs. 2 and 3 are sectional views of the tube of Fig. 1 taken on lines2-2 and 3--3 of Fig. 1, respectively;

Figs. 4, 5 and 6 are explanatory views of the magnetic fieldconfiguration in the tubeof Fig. l;

Fig. 7 is a graph illustrating the permeability characteristic of atypical ferrite for positive and negative circularly polarized waves;

Fig. 8 is a graph illustrating the relative attenuation of forward andbackward waves in the wave propagating structure of the tube of Fig. 1;

Figs. 9 and 10 are respectively a sectional and an elevational viewillustrating an embodiment of a magnetic shunt for the ferrite membersof the tube of Fig. 1;

Fig. 11 is a sectional view illustrating a modification of theembodiment of Fig. 1;

Fig. 12 is a graph illustrating the relative bidirectional attenuationcharacteristic for the Wave propagating structure of the tube of Fig.l1; and

Figs. 13 and 14 are diagrammatic sectional views of an alternateembodiment of the device of the present invention.

Referring now to the drawings, there is illustrated in Fig. l anembodiment of the present invention comprising an evacuated envelope 10,an electron gun 12 disposed within the envelope at the left extremitythereof as viewed in the figure for producing an electron stream, afolded waveguide structure 14 for propagating an electromagnetic signalwave to be amplified along a predetermined path for the electron stream,and a solenoid 16 disposed concentrically about the path for producing amagnetic field along the longitudinal axis of the tube.

More particularly, electron gun l2 comprises a cathode 20 with itsassociated heater 22 for providing an electron emitting surface, afocusing electrode 24, and an accelerating anode 26. Heater 22 isenergized by a connection across a battery 28, one terminal of which isconnected to cathode 20. In operation, the cathode 20 is maintained at apotential of the order of 2000 volts negative with respect to ground.This is accomplished by means of a connection from the cathode 20 to thenegative terminal of a battery 30, the positive terminal of which isconnected to ground. The focusing electrode 24 provides a conductivesurface of revolution about the path of the electron stream atapproximately 67.5 thereto. In accordance with this conguration,electrode 24 is maintained at the same potential as that of cathode 20by means of a connection therebetween. Accelerating electrode 26 isdisposed concentrically about the path of the electron stream to theright of and adjacent the focusing electrode 24, as shown. Electrode 26is maintained at a potential of the order of 200 volts positive withrespect to ground by means of a suitable connection to battery 32.

Folded waveguide structure 14 for propagating an electromagnetic wavealong the path of the electron stream comprises a length of rectangularwaveguide 40 that is periodically folded back and forth across the pathof the electron stream. Suitable apertures are disposed in the waveguidewalls to enable the stream electrons to proceed along the path. Theelectric field within the Waveguide is caused to concentrate about thepath of the electron stream by means of ferrules 42 which connectadjacent apertures of the folded waveguide structure and protrudeslightly inwards into the guide. Annular rings #i3 and d4 are disposedabout the periphery of the first and last apertures in the waveguidealong the path of the electron stream, respectively, to provide similarprotrusions into the guide from the end apertures. Dielectric seals 46,47 are disposed across the input and output ends of the waveguide,respectively, as shown so is to enable the envelope 10 to be evacuated.The waveguide 40 is maintained at ground potential by suit- ;hleconnections thereto.

A collector electrode 50 for intercepting and collecting the electronstream is disposed at the extremity of its path farthest from theelectron gun 12. Electrode is maintained at a potential of the order offrom 100 to 200 volts positive with respect to ground so as to minimizesecondary electron emission from its surface exposed to bombardment bythe electron stream. The above is accomplished by a connection to thepositive terminal of a battery 5l, the negative terminal of which isconnected to ground.

ln accordance with the present invention, ferrite slabs 60, 61 aredisposed on alternate sides of the electron stream for at least oneperiod of the folded waveguide structure 14. Sectional views indicatingrepresentative locations of ferrite slabs 60, 6l are illustrated inFigs. 2 and 3. Referring to these figures, ferrite slabs 60, 6l arespaced distances a and b, respectively, from the walls nf the waveguidesection 40. The dimensions a and b of the slabs 60, 61, respectively,may both equal approximately 25% of the width of the waveguide formaximum unidirectional attenuation. Alternatively, if it is desired thatthe power dissipated in each of the ferrite slabs 60, 61 besubstantially equal, the distances a and b may be made to progressivelyincrease for each half-period of the waveguide section 40 but retainedwithin the range of 10% to 40% of the waveguide cross section. On theother hand, the attenuation of an electromagnetic wave propagated byfolded waveguide section 14 may be made bidirectional by making each ofthe distances a and b equal to zero. ln this latter case, the selectiveattenuation characteristic of the ferrite material is made to occuroutside of and adjacent to the operating frequency range as will behereinafter explained.

Ferrite slabs 60, 6l have a thickness of the order of 0.025 inch andpreferably extend completely across the narrow dimension of thewaveguide 40 coextensive with the portion thereof normal to the path ofthe electron stream. The exposed edges of the ferrite slabs 60, 61 areslightly tapered, as shown in Fig. 1, so as to minimize their tendencyto reflect portions of electromagnetic waves propagated through theguide. Ferrite slabs 60, 61 may be composed of, for example, ferritematerial known commercially as Ferramic A, Ferramic G, Ferramic R-l,Ferroxcube 104, or Ferroxcube 106.

The solenoid 16, for producing the longitudinal magnetic field, isconnected across an adjustable potential source 54. The voltage outputof source 54 is adjusted so that solenoid 16 develops a magnetomotiveforce along the length of the tube depending upon the frequency ofoperation and the type of ferrites used which may be of the order of1500 oersteds. In accordance with the present invention, this magneticfield is employed to both focus and direct the electron stream along itspath and to produce transverse magnetic fields through the ferrite slabs60, 61.

To illustrate more clearly the magnetic field portion of anelectromagnetic wave propagated along the path of the electron stream,reference is made to Fig. 4, which shows a developed portion of thefolded waveguide section 40 of Fig. l. In this figure, the instantaneousmagnetic field for an electromagnetic field being propagated along thedeveloped waveguide section 40 is shown by dash lines 70. In theoperation of the tube of the present invention, magnetic fields of thisconfiguration are propagated along the folded waveguide section 40 at avelocity designated as the phase velocity of the wave.

In order to consider the characteristics of the magnetic fieldsrepresented by dash lines in the region occupied by ferrite slab primee. g. 61', the field will be viewed from a single point within the slabsuch as, for example, at point A. It is seen that the portion of themagnetic field that will intercept point A for the particular instantshown may be represented by vectors a, b c-and 'is-fas il lustrated inFig. 4. As the wave is propagated along the waveguide section 40, pointA will be successively cut by these points of the magnetic field. Thus,as viewed from point A, it is seen that, as shown in Fig. 5, themagnetic field appears as a vector that rotates in a clockwise directionat an angular velocity w1. When the moment of this rotating vectorcoincides with the direct-current magnetic field through the ferriteslab 61', the component of the wave described is said to be positivecircularly polarized. Alternatively, if the moment of the rotatingvector is in an opposite direction, the particular component describedwould be negative circularly polarized. Similarly, point B within slab60" sees successive magnetic vectors e, f, g, and h as indicated in Fig.4. As before, these vectors appear at point B as a single vectorrotating in the counter-clockwise direction at an angular velocity w2 asillustrated in Fig. 6.

In accordance with the present invention, a direct-current magneticfield is produced in opposite directions through ferrite slabs 60 and 61with respect to the direction in which the electromagnetic wave ispropagated through the developed waveguide structure. This directcurrentmagnetic field is directed in a manner such that the magnetic fieldcomponents of the wave being propagated in the forward direction will benegative circularly polarized. In that the magnetic field interceptingferrite slabs 60, 61 rotates in opposite directions, it is apparent thatin order to have components of the propagated wave on both sides of thewaveguide section 40 polarized in the same direction it is necessarythat the direct-current magnetic field through ferrite slabs 60 be in adirection opposite to that through slabs 61. In accordance with thepresent invention, this is accomplished by folding back-to-back thesuccessive portions of the waveguide containing ferrite slabs 60 and 61which are disposed on opposite sides thereof.

As is generally known, the permeability characteristics for positive andnegative circularly polarized waves of ferrite material are different.More particularly, the permeability for the negative circularlypolarized wave is substantially constant, whereas the permeability forthe positive circularly polarized wave goes through resonance forchanges in frequency or magnetic field. During this resonance the powerdissipated in the material increases substantially, thereby making thepermeability for a positive circularly polarized wave a complexquantity. A typical permeability characteristic for a ferrite materialis shown in Fig. 7 wherein ,L represents the permeability' for thenegative circularly polarized wave and p=jt-jn" represents thepermeability for a positive circularly polarized wave. As illustrated inthis figure, lines 74, 76 and 78 represent the variation in /.t y. andn", respectively, versus frequency for circularly polarized waves. Themagnitude of n", i. e. the imaginary part of a+, determines the extentto which the positive circularly polarized wave will be attenuated. Asshown in the figure, maximum attenuation of the positive circularlypolarized wave occurs at the resonant frequency. The point at which thisresonance occurs is both a function of the frequency of the circularlypolarized wave as seen by the ferrite material and the strength of thedirect-current magnetic field through the ferrite.

Thus, in the operation of the device of the present invention, thecurrent through solenoid 16 which determines the direct-current magneticfield is adjusted by means of adjustable potential source 54 to producea magnetic field within the ferrite material corresponding to resonanceat the operating frequency.

In operation of the device of the present invention, the function of theferrite slabs 60, 61 is to attenuate electromagnetic wave energy beingpropagated by the waveguide section 40 in the direction from thecollector 50 to electron gun 12, i. e. in the backward direction,without appreciably attenuating waves being propagated in the forwarddirection. The relative attenuation characteristics for a forward andbackward wave for the device of the present invention are shown in Fig.8 as lines 80 and 82, respectively.

As is evident from Fig. 8, when a single magnetic field strength is usedthrough the ferrite slabs 60, 61, the attenuation of backward wavesoccurs for only a comparatively narrow range of frequencies. Thisattenuation range may be broadened by causing resonance within theferrite slabs 60, 61 to occur at different frequencies within the bandwherein it is desired to attenuate the backward waves. This may beaccomplished by employing different types of ferrite material or,alternatively, by developing different magnetic fields within each ofthe ferrite slabs 60, 61. The magnetic fields within the ferrite slabsmay be varied, for example, by employing slabs of different thicknessesor magnetic shunts 84, 86 shown in Figs. 9 and 10. In the latter case,the magnetic shunts 84, 86 are composed of a material which presents alow reluctance to the magnetic field produced by solenoid 16 so as toshunt a portion of the field around the ferrite slabs 60 and 61.

ln an alternative embodiment of the present invention, ferrite slabs 90and 92 as shown in Fig. 11 are disposed in contact with the walls withinthe portions of the folded waveguide section 40 normal to the path ofthe electron stream. When disposed within a waveguide in this manner,both forward and backward waves are attenuated by substantially equalamounts. This attenuation, however, is a maximum at frequenciescorresponding to resonance for the ferrite slabs 90 and 92. Inaccordance with the present invention, each of the ferrite slabs 90 and92 are of at least two different thicknesses. Thus, when energized bythe magnetic field produced by solenoid 16, different magnetic fieldintensities are developed within each of the slabs 90 and 92 whichproduce resonance at frequencies f1 and f2 which are adjacent theoperating frequency range of the tube. In this manner, attenuation ofboth forward and backward waves is effectuated in the regions adjacentthe operating frequency range of the tube as indicated, for example, bylines 94, 96 of Fig. 12 thereby minimizing any tendency of the tube tobreak into self-oscillation due to a poor impedance match of foldedwaveguide structure 14 with the input and output circuits in theseranges. It is evident that this selective attenuation may be produced bymagnetic shunts or by the use of different ferrites in the mannerpreviously described.

In another embodiment of the present invention illustrated in Fig. 13, ahelical wave propagating structure 100 is employed in lieu of the foldedwaveguide structure 14 of the device shown and described in connectionwith Fig. 1, the remaining elements being substantially the same.Helical wave propagating structure 100 comprises a length of rectangularwaveguide 102 which is edge-wound about the path of the electron streamat a uniform pitch. The wall of the waveguide within the evacuatedenvelope and adjacent the path of the electron stream is removed so thatan electromagnetic wave propagated within each turn of the waveguidewill combine to form a wave capable of interacting with the electronstream.

In accordance with the present invention, a ribbon helix 104 of ferritematerial is disposed within the waveguide 102 coextensive with theedge-wound portion thereof and spaced from the outer wall 106. Ingeneral, the distance between the ribbon helix 104 and the outer wall106 of the waveguide may be from 10% to 30% of the width of the guide toproduce optimum unidirectional attenuation. In operation, the magneticfield produced by solenoid 16 provides focusing for the electron streamand in addition, constitutes the field which energizes the ferrite helix104 in a direction transverse to the waveguide 102. As before, voltagesource 54 is adjusted to produce a magnetic field necessary to effectresonance in the ferrite material at the desired frequency. In the eventthat it is desired to effect resonance over a broad range offrequencies, the thickness of the ribbon helix 104 could be tapered tocause the magnetic field within the ferrite material to progressivelyincrease or decrease.

What is claimed is:

1. A traveling-wave tube comprising a waveguide structure having aperiodic relationship with respect to a predetermined path wherebyelectromagnetic waves capable of being propagated by said waveguidestructure form concomitant waves along said path having velocitiessubstantially less than the velocity of light, at least one ferritemember disposed within said waveguide structure, said ferrite memberbeing composed of longitudinal segments parallel to said predeterminedpath and transverse to the direction of propagation of saidelectromagnetic waves through said waveguide structure, means forproducing an electron stream, and means for producing a predeterminedlongitudinal magnetic field through said waveguide structure andparallel to said predetermined path to constrain said electron streamtherealong and to develop a magnetomotive force across said longitudinalsegments to attenuate at least a portion of the circularly polarizedcomponents of said electromagnetic waves.

2. A traveling-wave tube comprising a conductive member providing alongitudinal rectangular enclosure for propagating electromagneticwaves, said longitudinal rectangular enclosure having periodicallyspaced apertures and being folded bach and forth in a manner to causesaid apertures to define a predetermined path, a plurality of ferritemembers disposed within said enclosure, said ferrite members beingconstituted of longitudinal segments parallel to said predetermined pathand transverse to the direction of propagation of said electromagneticwaves through said longitudinal rectangular enclosure, means forproducing an electron stream, and means for producing a predeterminedlongitudinal magnetic field through said conductive member and parallelto said predetermined path to constrain said electron stream therealongand to develop a magnetomotive force across said longitudinal segmentsto attenuate at least a portion of the circularly polarized componentsof said electromagnetic waves.

3. A traveling-wave tube comprising a conductive member providing alongitudinal rectangular enclosure for propagating electromagneticwaves, said longitudinal rectangular enclosure having periodicallyspaced apertures through the broad sides thereof, and being folded backand forth in a manner to cause said apertures to define a predeterminedpath, a plurality of ferrite slabs disposed on alternate sides of saidpath within said longitudinal enclosure, said ferrite slabs beingconstituted of longitudinal segments disposed parallel to said path andtransverse to the direction of propagation of said electromagnetic wavesthrough said longitudinal rectangular enclosure, means for producing anelectron stream, and means for producing a predetermined longitudinalmagnetic field through said conductive member and parallel to saidpredetermined path to constrain said electron stream therealong and todevelop a magnetomotive force across said longitudinal segments tounidirectionally attenuate said electromagnetic waves.

4. The traveling-wave tube as defined in claim 3 wherein said ferriteslabs are of different thicknesses whereby ditferent magnetic fieldstrengths are produced within individual ones of said longitudinalsegments of said ferrite slabs, thereby to unidirectionally attenuatesaid electromagnetic waves over a broad band of frequencies.

5. The traveling-wave tube as defined in claim 3 Wherein said ferriteslabs are spaced from 10% to 40% of the width of said rectangularenclosure from the nearest respective side thereof.

6. The traveling-wave tube as defined in claim 3 including additionalmeans for providing a magnetic shunt across the longitudinal segments ofat least one of said ferrite slabs, thereby to unidirectionallyattenuate said electromagnetic waves at selected frequencies.

7. A traveling-wave tube for amplifying microwave signals over apredetermined frequency range, said tube comprising a conductive memberproviding a folded longitudinal rectangular enclosure for propagatingelectromagnetic waves, `said longitudinal rectangular enclosure havingperiodically spaced apertures through the broad sides thereof to definea predetermined path, at least one ferrite member having a surfacedisposed contiguous to a narrow side of said rectangular enclosure,means for producing an electron stream, and means for producing apredetermined longitudinal magnetic field through said conductive memberand parallel to said predetermined path to constrain `said electronstream therealong and to develop a magnetomotive force across saidferrite member to bidirectionally attenuate electromagnetic Waves havingfrequencies adjacent said predetermined frequency range.

8. A traveling-wave tube comprising a slow-wave structure forpropagating electromagnetic waves, said slow-wave structure including arectangular waveguide edge-wound at a uniform pitch about apredetermined path, said waveguide having a longitudinal aperture in theside thereof adjacent said path, and a tape helix composed of ferritematerial disposed within said edgewound waveguide and spaced from theside thereof farthest from said predetermined path; means for producingan electron stream; and means for producing a predetermined longitudinalmagnetic field through said slow-wave structure and parallel to saidpredetermined path to constrain said electron stream therealong and todevelop a magnetomotive force across the width of the tape of said tapehelix to unidirectionally attenuate said electromagnetic waves.

9. The traveling-wave tube as defined in claim 8 wherein said tape helixis spaced from 10% to 40% of the width of said rectangular waveguidefrom the side thereof farthest from said predetermined path.

l0. The traveling-wave tube as defined in claim 8 wherein the thicknessof said tape helix is progressively increased thereby tounidirectionally attenuate said electromagnetic waves over a broad rangeof frequencies.

References Cited in the file of this patent UNITED STATES PATENTS2,644,930 Luhrs et al. July 7, 1953 2,653,270 Kompfner Sept. 22, 19532,660,689 Touraton et al. Nov. 24, 1953 2,672,572 Tiley Mar. i6, 1954

